Previously, in electrophotographic devices such as plain paper copiers (PPCs), laser printers, LED printers and liquid crystal printers, images have been formed on photoreceptors of the rotary drum type through an image forming process comprising charging, exposure and development, and transferred to transfer members, followed by fixing, thus obtaining duplicated copies. As the photoreceptors used in these devices, inorganic photoreceptors such as selenium, arsenic-selenium, cadmium sulfide, zinc oxide and a-Si photoreceptors are employed, but organic photoreceptors (OPCs) inexpensive and excellent in productivity and waste disposal are also actively studied and developed. In particular, so-called function separation type photoreceptors in which charge generating layers are laminated with charge transporting layers are excellent in electrophotographic characteristics such as sensitivity, charge property and repetition stability thereof, so that various function separation type photoreceptors have been proposed and have come in practice.
However, the characteristics required for electrophotographic photoreceptors, particularly the durability has yearly become severe, and to the problems of wear and damage of surface layers due to repeated use, particularly wear and damage of surface layers significantly promoted by the use under contact charging, and oxidation deterioration of surface layers caused by oxidizing gases such as ozone generated from corona charging units, techniques necessary for improvement in the durability have been continuously studied. As a method for solving these problems of surface layers, a method of forming a surface protective layer on a charge transporting layer is proposed, the surface protective layer being mainly composed of a crosslinking hardenable resin such as an organic polysiloxane (JP-A-54-148537 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")).
A proposal has been also made for an approach which comprises forming a surface protective layer comprising an oxidation inhibitor incorporated in a hardening resin depending on the required durability to enhance the chemical durability of the surface layer against ozone, nitrogen oxide, etc. produced by corona discharge (JP-A-63-18354).
However, when the surface protective layer is formed of the crosslinking hardenable resin alone, it becomes an insulating layer, which sacrifices electrophotographic characteristics of a photoreceptor. Specifically, when the surface protective layer becomes an insulating layer, the illuminated part potential on exposure is increased. Accordingly, the development potential margin is narrowed, or the residual potential after charge elimination is elevated. There has been therefore the problem that the image density is lowered, particularly when printing is repeated for a long period of time.
As a method for improving these electrophotographic characteristics, a method is proposed in which a fine conductive metal oxide powder is dispersed in a surface protective layer as a resistance controlling material (JP-A-57-128344).
This method restrains a reduction in the electrophotographic characteristics of a photoreceptor to substantially improve the above-mentioned problem. However, the value of resistance of the metal oxide used as the fine conductive powder largely depends on the humidity of the environment. This method has therefore the substantial problem that the surface resistance of the photoreceptor is reduced, particularly under the circumstances of high temperature and humidity, to blur an electrostatic latent image, which causes the image quality to be largely deteriorated.
Further, as another technique for improving the electrophotographic characteristics, a method is proposed in which a charge transporting material is dispersed in a binder resin, and then, the binder resin is hardened to form a surface protective layer (JP-A-4-15659).
This method removes the humidity dependence of the surface resistance of the photoreceptor, thereby solving the problem of the image quality. However, the addition of the charge transporting material, namely a low molecular weight component, inhibits the hardening reaction of the binder resin to decrease the mechanical strength of the surface protective layer. Accordingly, even if a crosslinking hardenable resin having a high mechanical strength is solely used, a substantial reduction in the mechanical strength of the surface protective layer can not be avoided, so long as the low molecular weight component is contained as the charge transporting material indispensable for improvement in the electrophotographic characteristics.
Then, a methods is proposed in which a functional group-containing charge transporting material is acted on or reacted with a binder resin, thereby improving the mechanical strength of a surface layer (JP-A-6-202354 and JP-A-5-323630).
According to this method, a sufficient mechanical strength can be obtained initially without reducing the electrophotographic characteristics of the photoreceptor. However, the use of the photoreceptor for a long period of time under the contact charging system or the scorotron charging system rapidly decreases the mechanical strength of the above-mentioned surface protective layer. This is considered to be caused by a strong external stress, such as severance of bonds of the resin of the surface protective layer by the application of the alternating current voltage in contact charging, or the oxidative decomposition of the charge transporting layer with ozone generated in scorotron charging.
Further, the prevention of abrasion merely by raising the mechanical strength of the surface protective layer is disadvantageous in that paper powder or toner attached to the surface of the photoreceptor can be easily fixed thereto, resulting in a drastic deterioration of image quality.
Moreover, when such a surface protective layer as described above is employed, the mechanical strength may be improved. However, a problem that a charge-generating material and a charge-transporting material is fatigued to be deteriorated due to a photoelectric current repeatedly passing through the photosensitive layer. This problem becomes marked as the printing resistance is improved and the number of sheets repeatedly printed is increased. Therefore, a charge-generating material and a charge-transporting material which are stable against a photoelectric current should be used to solve the problem.